[0001] The invention relates to a method for operating a system for
electrolysis, for example for obtaining hydrogen or another gaseous
10 electrolysis product, in which at least one electrolysis device is supplied
with electrical energy via a power converter, and to such a system.
PRIOR ART
15 [0002] In order to obtain hydrogen, so-called electrolysis can be used, in
which, for example, water is split up by electrical energy into oxygen and hydrogen, i.e., gaseous electrolysis products or products of the underlying redox reaction. Water electrolysis is also referred to here. What are known as alkaline water electrolysis (or AEL for "alkaline electrolysis") or so-called
20 proton exchange membrane electrolysis (or PEM electrolysis for "Proton
Exchange Membrane" electrolysis) then also come into consideration here. The fundamentals for this are known per se, e.g., from "Bessarabov et al: PEM electrolysis for Hydrogen production. CRC Press." In addition, there is also the so-called SOEC ("Solid Oxide Electrolysis Cell") and AEM
25 ("Anion Exchange Membrane") electrolysis, as well as proton-conducting
high-temperature electrolysis, PCEs (Proton Ceramic electrolysers), e.g., approximately 400°C to 700°C, see, for example, Vøllestad et al. “Mixed proton and electron conducting double perovskite anodes for stable and efficient tubular proton ceramic electrolysers” in Nature Materials, 2019.
30
[0003] In particular, those electrolysis technologies that take place at low temperatures, i.e., PEM, AELL and AEM electrolysis, are suitable due to the possibilities of flexible operation for supporting the transition of energy production to renewable energy. An operation of a corresponding system
2

for electrolysis is suitable for this purpose, in which the required electrical
energy is obtained from, for example, a power supply grid, such as the
public power supply. However, so-called island grids also come into
consideration if, for example, such a system is operated (directly) at a wind
5 turbine or a wind farm with a plurality of wind turbines.
[0004] However, here, problems may occur due to retroactive effects on the power supply grid, and these retroactive effects are usually the stronger the smaller the power supply grid is.
10
[0005] The object of the present invention is therefore to specify improved possibilities for operating a system for electrolysis.
DISCLOSURE OF THE INVENTION
15
[0006] This task is solved by a method for operating a system for electrolysis and by such a system with the features of the independent claims. Embodiments are the subject matter of the dependent claims and of the description below.
20
ADVANTAGES OF THE INVENTION
[0007] A method according to the invention serves to operate a system for electrolysis to obtain at least one gaseous electrolysis product, in which at
25 least one electrolysis device is electrically connected to a power converter
via a direct-voltage circuit (also referred to as a direct-voltage intermediate circuit). The power converter in turn is connected with an alternating-voltage circuit in order to supply the at least one electrolysis device with electrical energy for its operation. The alternating-voltage circuit can
30 (directly) be a power supply grid, but it is typical and expedient when the
alternating-voltage circuit is electrically connected to a power supply grid by means of a transformer. Thus, the typically very high alternating voltage in the power supply grid (at least when used on an industrial scale, a high
3

voltage is typical) can be transformed down to a lower, required value of the alternating voltage.
[0008] The public power supply or a public power supply grid can be used
5 as the power supply grid. However, it is also preferred if an island grid is
used as the power supply grid, i.e., a (self-contained) power supply grid, such as a wind turbine or a wind farm with a multitude of wind turbines.
[0009] The power converter is necessary in order to convert the alternating
10 voltage, as is typical of a power supply grid, into the direct voltage required
for operating the electrolysis device(s). In this sense, a so-called inverter
or AD-DC-converter can be used as the power converter. At this point,
however, it should be noted that, in principle, the conversion of direct
voltage into alternating voltage may also be possible with such a power
15 converter. Typically, such a power converter has semiconductor switches,
such as IGBTs or thyristors or MOSFETs, which are correspondingly connected, usually in a so-called bridge circuit, and then controlled to convert the alternating voltage into a direct voltage.
20 [0010] Although, within the scope of the present application, the system is
mainly described with respect to (only) one electrolysis device, such a system may also have a plurality of such electrolysis devices that are electrically connected to the power converter via the direct-voltage circuit or a direct-voltage circuit. It is also conceivable that, in addition or
25 alternatively, further electrolysis devices are electrically connected via
another direct-voltage circuit and another (similar) power converter and then, via this, to the transformer.
[0011] Furthermore, the system can preferably be used for water
30 electrolysis, i.e., for obtaining hydrogen as a gaseous electrolysis product.
In particular, the types of water electrolysis already mentioned at the outset come into consideration here. Likewise, however, the system can, additionally or alternatively, also be used for carbon dioxide electrolysis (CO2 electrolysis) (this serves in particular to obtain CO or carbon
4

monoxide as a gaseous electrolysis product) and/or for co-electrolysis (this
serves in particular to obtain synthesis gas as a gaseous electrolysis
product), in which carbon dioxide, or carbon dioxide and water, are
converted into various products (in particular gaseous electrolysis
5 products), such as CO, synthesis gas or also ethylene, ethanol, format.
Chlorine-alkali electrolysis also comes into consideration. In addition, the
system can particularly preferably be used for low-temperature electrolysis
and/or for mid-temperature electrolysis and/or high-temperature
electrolysis, as described in part at the outset. For example, the EPM, AEL
10 and AEM are operated as low-temperature electrolysis more typically at
less than 100°C, although temperatures of up to 130°C are also possible and sometimes even very efficient. In the case of medium-temperature electrolysis, steam (and no liquid water) is generally used, temperatures between 150°C and 400°C being considered, for example. High-
15 temperature electrolysis usually involves electrolysis using ceramic
membranes, e.g., SOEC or the HT-PEM described, in a temperature range above 600°C. The individual electrolysis devices are then designed accordingly for this purpose. However, the specific type of electrolysis carried out with the system is less relevant to the present invention, as will
20 become apparent from the following explanations; in particular, the present
invention can be used with any type of electrolysis based on water and/or carbon dioxide as the feedstock and also for chlorine-alkali electrolysis (this is used in particular to obtain chlorine as a gaseous electrolysis product).
25
[0012] However, when the electrolysis device is supplied with electrical
energy via the converter, feedback or repercussions occur in the
alternating-voltage circuit or the power supply grid due to the operation of
the converter and the control of the semiconductor switches it contains.
30 These feedbacks or back-effects are primarily based on the harmonic
oscillations (i.e., fundamental oscillation and in particular harmonics) in the alternating voltage, which arise from or in the rectification of the alternating voltage. The voltage regulation then typically takes place by means of a
5

phase-cut control, but this in turn amplifies the (undesired) harmonic oscillations.
[0013] In the proposed method, the power converter is now operated by
5 means of a vibration package control. In the case of the vibration package
control, also referred to as wave packet control, in contrast to the phase-cut control, a pulse is connected only in or at least close to zero crossings. For this reason, this type of control system is also referred to as "Zero Crossing Control." The switching process of a semiconductor switch thus
10 takes place when the applied vibration of the alternating voltage is zero, or
a switching process already triggered previously is delayed until such a zero crossing occurs. Current and voltage transients and thus harmonics are thereby at least largely avoided. In particular, a reduction in the voltage (with regard to the mean value or effective mean value) is thus also
15 possible.
[0014] In this vibration package control, in particular, a full-wave control or
a half-wave control can be used. In the case of full wave control, all
periods of the frequency of the alternating voltage are always switched on
20 or off. As a result, no identical components occur in the power
consumption. Half-waves can also be connected to increase the continuity of the effective voltage. If direct-current components are to be avoided, it should be ensured that negative and positive half-waves occur equally frequently.
25
[0015] By using this vibration package control and the associated
prevention or at least reduction of feedback or effects into the power
supply grid, previously necessary filters (e.g., low-pass filters which filter
out the frequencies of these harmonics) can be avoided. The efficiency of
30 the operation of the system is thus increased. In addition, due to the now
lower back-effects in the power supply grid, more and/or larger systems can also be operated for electrolysis via a power supply grid, because no or hardly any back-effects occur, which could cause disturbances
6

elsewhere. The transition of the energy extraction to renewable energies already mentioned at the outset can thus be supported even better.
[0016] As already mentioned, a transformer is usually used to transform
5 down the alternating voltage of the power supply grid to a value suitable
for the power converter. In this case, it is then preferred if the transformer is operated using a tap changer.
[0017] Tap changers for transformers, in particular power transformers,
10 serve to adjust the transmission ratio (the amplitude of the alternating
voltage between input voltage and output voltage). For this purpose, the
winding of the transformer on its upper or lower voltage side usually
comprises a trunk winding and a regulating or step winding with a plurality
of taps which are guided to the tap changer. The power control when
15 connected in parallel can also be realized via the tap changer.
[0018] Tap changers are divided into on-load tap changers (OLTC) and no-load tap changers (NLTC), or DETC for de-energized tap changers or OCTC for off-circuit tap changers, wherein these terms are synonymous.
20
[0019] On-load tap changers are used for uninterruptible switching under load and can be divided into load selectors and load switches. Depending on the operating currents to be handled and the installation location in the transformer circuit, tap changers can be installed in a single-phase or
25 three-phase manner. This means that a tap changer column switches
either one or three phases. Three single-phase tap changers require more space than one three-phase tap changer. The use of three-phase tap changers usually presupposes the installation location at the star point of a star connection. Single-phase switches are usually required for larger
30 currents, higher switching power, or for use in a delta circuit.
[0020] No-load tap changers in principle fulfill the same tasks as on-load tap changers but can only be adjusted without load or voltage. No-load tap changers are usually executed with a few stages and are often actuated
7

only manually, although automated actuation is of course also possible. However, they are largely maintenance-free.
[0021] Due to the avoidance of feedbacks by the vibration package control
5 used, no such retroactive effects also occur in the transformer and a
particularly efficient and interference-free switching operation is made possible by means of the tap changer. The available, settable voltage range can be increased without (negative) effects on direct-current ripples.
10 [0022] The aforementioned full-wave control when providing the direct
voltage by means of the power converter basically allows a voltage range of 0% to 100% of the input voltage as output voltage, if no negative effects on direct-current ripples are to be allowed, however, a voltage range of 70% to 100%, preferably 80% to 100%, is expedient (thus in particular a
15 direct-current ripple can be kept low during electrolysis). A tap changer
basically allows voltage ranges without limits at the bottom or above; however, a voltage range of 90% to 110% is economically preferred. These voltage ranges or operating ranges are sufficient to compensate for aging effects during the electrolysis or of an electrolysis device and to
20 keep the extraction or production rate of, for example, hydrogen constant
over the service life (and thus also its previous operating time) of the electrolysis device. In particular, however, the electrolysis device can also always be operated flexibly. In this respect, it is therefore particularly expedient to achieve a nominal capacity (ultimately corresponds to the
25 extraction rate) of the electrolysis device for the gaseous electrolysis
product even in the case of degradation over the service life.
[0023] The background here is that the voltage required for operating an
electrolysis device at a certain production rate increases over time so that
30 the voltage provided must be increased over time in order to keep the
production rate constant (if possible). A certain flexibility of the operation is thus made possible, i.e., the production rate can be increased or reduced. Alternatively or additionally, it is also preferred to completely switch off or on individual stacks of an electrolysis device and/or individual electrolysis
8

devices (especially in the case of a plurality of electrolysis devices) as required. Switching on or off individual stacks further increases the working range or enables an adaptation of the load range.
5 [0024] The subject matter of the invention is furthermore a system for the
electrolysis to obtain at least one gaseous electrolysis product, with at least one electrolysis device and one power converter, wherein the at least one electrolysis device is electrically connected to the power converter via a direct-voltage circuit, wherein the power converter is electrically
10 connectable or connected to an alternating-voltage circuit in order to
supply the at least one electrolysis device with electrical energy for operation thereof, wherein the system is configured to operate the power converter by means of a vibration package control. With regard to the advantages and further preferred embodiments of the system, reference is
15 made to the statements relating to the method, which apply here
accordingly, in order to avoid repetition.
[0025] The invention is explained in more detail below with reference to the
accompanying drawing, which shows a system according to a preferred
20 embodiment of the present invention.
BRIEF DESCRIPTION OF THE DRAWING
[0026] Figure 1 schematically shows a system according to the invention in
25 a preferred embodiment.
[0027] Figure 2 schematically shows the operation of a vibration package control as used in the context of the present invention.
30 [0028] Figure 3 schematically shows voltage curves for the operation of an
electrolysis device that may be part of a system according to the invention.
DETAILED DESCRIPTION OF THE DRAWING
9

[0029] Figure 1 schematically shows a system 100 according to the
invention in a preferred embodiment. The system 100 is used for
electrolysis and has a transformer 110, an alternating-voltage circuit 120, a
power converter or inverter 130, a direct-voltage circuit 140 and, for
5 example, two electrolysis devices 150 and 160. It goes without saying that
even only one electrolysis device can be provided, or that even more electrolysis devices can be provided.
[0030] The transformer 110 has a tap changer 110, for example an on-load
10 tap changer, and is electrically connected on the input side (or
corresponding terminals) to a power supply grid 200 and on the output
side (or corresponding other terminals) to the alternating-voltage circuit
120. The alternating voltage provided by the power supply grid 200 can
thus be transformed down by means of the transformer 110, wherein the
15 transformation ratio can be changed by using the tap changer 111.
[0031] The alternating-voltage circuit 120 is then electrically connected to the power converter 130 or corresponding terminals or input terminals of the power converter 130. The power converter 130 in turn is electrically
20 connected to the direct-voltage circuit 140 via corresponding connections
or output connections. The power converter 130 also has a control unit 131 by means of which semiconductor switches provided in the power converter can be activated accordingly, i.e., opened and closed, in order to rectify the alternating voltage.
25 The electrolysis devices 150 and 160 are in turn electrically connected to
the direct-voltage circuit 140.
[0032] In this way, electrical energy for operating the system 100 or the
electrolysis devices 150, 160 comprised thereof can be provided by means
30 of the power supply grid 200. By way of example, the electrolysis device
150 is designed for water electrolysis, in which water a is supplied and split into a plurality of stacks (only indicated) and hydrogen b and oxygen c are obtained and discharged as gaseous electrolysis products and optionally stored. It is also conceivable to (further) clean the gaseous
10

electrolysis product, for example by drying and/or removing other gases.
The electrolysis device 160 may have the same design or may also be
different. As already mentioned at the outset, the specific type of
electrolysis device is less relevant to the present invention; rather, the
5 operation of the power converter 130 and possibly of the transformer 110
is important.
[0033] As mentioned, for operating the system 100, the power converter 130 or the semiconductor switches contained therein are controlled, in
10 particular by means of the control unit 131, in such a way that the
semiconductor switches always switch at or near a zero crossing of the relevant, applied vibration of the alternating voltage. The power converter 130 is thus operated by means of a vibration package control. The exact switching time does not have to be exactly at the zero crossing but can
15 instead be up to 5% or up to 10% (relative to a period duration of the
oscillation) before or after, for example.
[0034] In this way, feedback into the alternating-voltage circuit 120 and
thus into the transformer 110 as well as the power supply grid 200 are
20 prevented. A filter for reducing such undesired harmonics or feedback, as
was previously necessary and shown in dashed lines in Figure 1, cf. reference sign 115, is thus no longer necessary.
[0035] Figure 2 schematically shows a control of the power converter with
25 the vibration package control and thus its operation, as used in the context
of the present invention. For this purpose, a voltage V is plotted over a time t, and vibrations or waves of the alternating voltage as they are present at the input of the power converter are shown.
30 [0036] For this purpose, t0 shows a vibration package duration of three full
or whole vibrations here by way of example; tE, a switch-on duration of two full or whole vibrations here by way of example. It is hereby only switched at zero crossings, i.e., e.g., at t=0, t=tE or t=t0, so that no undesired
11

harmonics can occur. In addition, this is only switched in the case of whole vibrations.
[0037] Figure 3 shows schematic and purely exemplary or generic voltage
5 curves for the operation of an electrolysis device, which can be part of a
system according to the invention and is shown as an example in Figure 1. For this purpose, a voltage V is applied above a current density I (instead, this can also be a density of hydrogen).
10 [0038] Curve V1 represents the relationship between the necessary
voltage V and the current density I achieved therewith at the beginning of the service life of the electrolysis device, whereas curve V2 represents the corresponding relationship at the end of its service life. It can be seen that as the service life increases, an increasingly higher voltage is required
15 here in order to achieve the same current density; the difference between
the start and end of the service life is denoted here by ∆V.
[0039] Absolute values of the voltages usually vary in practice depending
on the electrolysis technology and the number of cells in the stack of an
20 electrolysis device. In this respect, as mentioned, only exemplary or
generic curves are shown here. A slope also varies depending on electrolysis technology, insofar as they are likewise shown here only by way of example or generically.
25 [0040] However, by means of the above-described system and the
proposed operation of such a system, it is possible to change the voltage applied to the electrolysis device and thus, for example, to select a lower voltage at the beginning of the service life, which is increased more and more over time in order to keep the current direction and thus also the
30 production rate constant (if possible).

WE CLAIMS

Method for operating a system (100) for electrolysis to obtain at
5 least one gaseous electrolysis product, in which system at least one
electrolysis device (150, 160) is electrically connected to a power
converter (130) by means of a direct-voltage circuit (140), wherein the
power converter (130) is connected to an alternating-voltage circuit (120)
in order to supply the at least one electrolysis device (150, 160) with
10 electrical energy for its operation,
wherein the power converter (130) is operated by means of a vibration package control.
2. Method according to claim 1, wherein a full-wave control or a half-
15 wave control is used in the vibration package control.
3. Method according to claim 1, wherein a full-wave control is used in
the vibration package control, and wherein a voltage range of 70% to
100% of the input voltage is used as the output voltage.
20
4. Method according to any of the preceding claims, wherein the
alternating-voltage circuit (120) is electrically connected to a power supply
grid (200) by means of a transformer (110).
25 5. Method according to claim 3, wherein the transformer (110) is
operated using a tap changer (111).
6. Method according to claim 4, wherein the transformer (110) is
operated using an on-load tap changer or a no-load tap changer as a tap
30 changer (111).
7. Method according to claim 4 or 6, wherein a voltage range of 90%
to 110% is used in the transformer with the tap changer.
13

8. Method according to any of claims 4 to 7, in which a public power
supply grid or an island grid is used as power supply grid (200).
9. Method according to any of the preceding claims, wherein a voltage
5 provided for the at least one electrolysis device is adapted, in particular
increased, as a function of a previous operating time.
10. Method according to claim 9, wherein the voltage provided for the at
least one electrolysis device is adapted as a function of a previous
10 operating time in order to achieve a nominal capacity (ultimately
corresponds to the extraction rate) of the electrolysis device for the gaseous electrolysis product, even in the case of degradation over the service life.
15 11. Method according to any of the preceding claims, wherein one or
more gaseous electrolysis products are discharged and, in particular, stored and/or purified.
12. Method according to any of the preceding claims, wherein one or
20 more stacks of the at least one electrolysis device are switched on and/or
off as required.
13. Method according to any of the preceding claims, wherein the
system (100) is used for water electrolysis to obtain hydrogen and/or for
25 carbon dioxide electrolysis to obtain carbon monoxide and/or for co-
electrolysis to obtain synthesis gas and/or for chlorine-alkali electrolysis to obtain chlorine.
14. Method according to any of the preceding claims, wherein the
30 system (100) is used for low-temperature electrolysis and/or for medium-
temperature electrolysis and/or high-temperature electrolysis.
15. System (100) for electrolysis to obtain at least one gaseous
electrolysis product, with at least one electrolysis device (150, 160) and
14

one power converter (130), wherein the at least one electrolysis device
(150, 160) Is electrically connected to the power converter (130) via a
direct-voltage circuit (140), wherein the power converter (130) is
electrically connectable or connected to an alternating-voltage circuit (120)
5 in order to supply the at least one electrolysis device (150, 160) with
electrical energy for its operation,
wherein the system (100) is configured to operate the power converter (130) by means of a vibration package control.
10 16. System (100) according to any of the preceding claims, which is
configured to perform a method according to any of claims 1 to 14.